Powertrain, generic (extended)

P24CB Reductant Tank Cap Switch Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

Some SCR diesels have a switch at the DEF filler neck that tells the engine controller when the cap is opened, so a refill can be recognised and the tank level and fluid quality reassessed. P24CB is the general circuit code for this switch: the controller sees a signal that fits neither the open nor the closed state. Audi documents it together with P24CC for the 2013 to 2015 Q7 TDI in bulletin 2051053: the microswitch at the AdBlue filler neck of the active tank tends to stick, above all in hot and humid weather, and then sets the codes after refilling while guided fault finding shows nothing. The fix is an improved switch, for which the tank has to come out. Short to ground or supply and intermittent faults have their own codes P24CD to P24CF.

Likely causes

Cap microswitch sticking, especially in hot and humid conditions
Corroded connector or chafed wire at the filler neck
Dried DEF crystals around the switch actuator

Symptoms

  • Malfunction indicator lamp on, often right after refilling DEF
  • Refill not recognised, DEF warnings may persist

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–600
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant tank cap switch

Related codes

P24CCP24CDP24CEP24CF

References: static.nhtsa.gov, static.nhtsa.gov, law.cornell.edu.